The present invention relates to an agent imparting impact resistance that imparts superior impact resistance to polylactic acid and other polyhydroxy carboxylic acids, and a polyhydroxy carboxylic acid having superior impact resistance that contains the agent.
Although polylactic acid and other polyhydroxy carboxylic acids are polymers that have biodegradability and moldability, polylactic acid and polyhydroxybutyrate in particular have problems in terms of being excessively brittle and difficult to form, thereby limiting their industrial applications.
Various studies have been conducted in order to improve the brittleness of polylactic acid, and the addition of plasticizer in particular is known as a typical polymer modification method. This method has been examined early on as a method for giving flexibility without impairing transparency.
As an example of a plasticizer for polylactic acid, a method is disclosed in U.S. Pat. No. 1,995,970 for increasing flexibility and tear strength by adding dibutylphthalate and nitrocellulose to polylactic acid. U.S. Pat. No.3,498,957 discloses a plasticizer used when melting that lowers the viscosity of polylactic acid during polymerization by adding glycol diester and dibasic acid diester during polymerization.
U.S. Pat. No. 5,180,765 discloses a method for softening polylactic acid by adding lactic acid oligomer and lactide to polylactic acid. This method, however, is known to have problems such as increased susceptibility to decreased heat resistance and hydrolysis of the polymer itself.
European Patent No. 226061 discloses a polylactic acid composition containing a plasticizer such as triethyl citrate as an application to medical materials.
In addition, Japanese Unexamined Patent Application, First Publication No. 2-117 discloses a polylactic acid composition that contains ester acetate as a plasticizer that is useful as a plasticizing technology of biomaterials such as medical films and rods for implanting in the body.
Japanese Unexamined Patent Application, First Publication No. 4-335060 discloses a composition containing polylactic acid and a plasticizer, and more particularly, a polylactic acid composition containing a general-purpose plasticizer for ordinary general-purpose resins such as phthalic acid ester, aliphatic dibasic acid ester, phosphate, hydroxy polyvalent carboxylate, fatty acid ester, polyvalent alcohol ester, epoxy plasticizer, and polyester plasticizer, or a mixture thereof.
Although polylactic acid can be softened with these techniques, the heat resistance of the plasticized polylactic acid considerably decreases, and although it is soft, it still has numerous shortcomings including the absence of impact strength, cracking during bending and a decrease in the molecular weight of the polylactic acid during kneading. In addition, vaporization of the plasticizer during processing and the problem of bleedout cannot be avoided in the case of these low molecular weight plasticizers.
In addition, polyester plasticizers have difficulty in obtaining adequate flexibility, and suffer from considerable bleedout during storage. Other high molecular weight plasticizers have also been reported, including polyesters such as polycaprolactone and polyethers. Japanese Unexamined Patent Application, First Publication No. 8-199052 discloses that polyethers are useful as plasticizers of polylactic acid, while Japanese Unexamined Patent Application, First Publication No. 8-283557 discloses that an aliphatic polyester composed of aliphatic carboxylic acid and aliphatic diol is useful as a plasticizer used for the purpose of softening polymers consisting primarily of polylactic acid.
In all of these cases, however, the amount that can be added is only able to slightly improve the impact strength of the polylactic acid, and when an attempt is made to increase softness considerably, it causes a decrease in the heat-resistant temperature and bleedout in the same manner as the case of a low molecular weight plasticizer, while transparency also decreases depending on the type and amount of plasticizer added. In addition, although elasticity can be given, there are disadvantages consisting of weakness to impacts and low crazing resistance.
Although Japanese Unexamined Patent Application, First Publication, No. 9-137047 discloses a polylactic acid composition in which polyester having a low melting point is copolymerized to polylactic acid followed by the addition of copolymer or homopolymer having a structure similar to the copolymer, the resulting composition does not allow the obtaining of large impact strength.
In addition, even if the melting point is 100xc2x0 C. or lower, aliphatic polyester having a high degree of crystallinity and high Tg exhibits low plasticizing effects and low impact resistance improvement effects even if a plasticizer is added. This suggests impact strength does not always result in improved impact strength of polyhydroxy carboxylic acids such as polylactic acid simply because of a similar melting point of additives or similar structure with the base polymer.
The problem to be solved by the present invention is to provide an agent imparting impact resistance that is able to provide impact resistance while being resistant to bleedout and maintaining superior flexibility and transparency, in particular, to provide an agent imparting impact resistance which is useful for polyhydroxy carboxylic acids. In addition, another problem to be solved by the present invention is to provide a polyester composition having superior impact resistance while being resistant to the occurrence of bleedout and maintaining superior flexibility and transparency, as well as provide a molded article using the composition.
As a result of earnest research conducted to solve the above problems, the inventors of the present invention found that a lactic acid polyester (III) having a lactic acid unit (I) and polyester unit (II) at a weight ratio within the range of 10:90 to 90:10, a weight average molecular weight of 10,000 or more, and a glass transition temperature of 60xc2x0 C. or below is able to impart impact resistance to polyhydroxy carboxylic acids while being resistant to bleedout and maintaining superior flexibility and transparency, thereby leading to completion of the present invention.
Namely, in order to solve the above problems, the present invention provides an agent imparting impact resistance (IV) for polyhydroxy carboxylic acids comprising a lactic acid polyester (III) having a lactic acid unit (I) and polyester unit (II) at a weight ratio within the range of 10:90 to 90:10, a weight average molecular weight of 10,000 or more, and a glass transition temperature of 60xc2x0 C. or below.
In addition, in order to solve the above problems, the present invention provides a polyester composition (VI) containing a polyhydroxy carboxylic acid (V) and an agent imparting impact resistance, the agent imparting impact resistance comprising a lactic acid polyester (III) having a lactic acid unit (I) and polyester unit (II) at a weight ratio within the range of 10:90 to 90:10, a weight average molecular weight of 10,000 or more, and a glass transition temperature of 60xc2x0 C. or below.
Moreover, in order to solve the above problems, the present invention provides a molded article composed of the above polyester composition (VI).
Moreover, in order to solve the above problems, the present invention provides a film composed of the above polyester composition (VI).